This was an unfortunate copypasta error that slipped in with 9836d6d.
The HAVE_SYSTEMD macro should only be defined based on the presence of
the library, not the unit files, since the units are useless without the
library support.

Add flags and libraries to the respective environment vars and let make
figure out the order on its own. This provides support for unmentioned
vars such as LDFLAGS implicitly, as the compilation rule isn't
explicitly defined.